Understanding Minimum Weight

Is your balance suitable to accurately weigh your samples?
Semi-micro balances (those that weigh with a readability or steps of 0.01mg) are frequently regarded as being overkill or needed only for extremely sensitive weighing applications- but are they, in fact, the proper tool for the job?
While a balance's readability does not equal its accuracy, typically a balance with a smaller readability will offer greater accuracy than a balance with a larger readability (for example, a balance capable of reading by 0.1mg will be more accurate than a 1mg balance).
Abiding by a minimum weight convention will ensure a relative uncertainty less than some specified value.  Adopting a minimum weight requirement is a good weighing practice to ensure that your samples are being weighed according to your acceptable criteria for accuracy. 

USP (United States Pharmacopeia) defines a weighing guideline that uses a minimum weight requirement; however, it is important to note that while this particular guideline is only relevant for pharmaceutical laboratories which must be in compliance with USP, it is only one example of a minimum weight guideline and is not the sole definition.  In general, by observing a minimum weight for that weight and all heavier weight values, a maximum measurement uncertainty can be ensured.
Let's say, for example, you wish to weigh samples with a weight of 0.05 g (50 mg), which balance would you choose?  A 1mg balance?  A 0.1mg balance?  The answer to that question requires that a maximum relative measurement uncertainty tolerance be defined.  Do you require a maximum uncertainty of 1% of your sample weight? 0.1%?
A minimum weight is determined by dividing the standard deviation of the repeatability (Rstd) of the balance by the required accuracy.  In some cases, a safety factor is applied (for example, USP uses 2 x Rstd).  To weigh a sample of 50mg, one might assume that a balance with a readability of 0.1mg is quite sufficient.  Is this correct?  Let's look at an example to find out.  If the maximum uncertainty tolerance is 0.1%, and the balance's Rstd is 0.1mg, then the minimum weight would equal 2*0.1mg / 0.001 or 200mg.  Thusly, a 0.1mg balance would be insufficient to meet the stated accuracy requirements for this example, and a balance with a readability of 0.01mg must be used.
In this case, a semi-micro balance such as the Explorer 5 Series from OHAUS would be needed to 'accurately' weigh 50mg samples. In most cases, the OHAUS Explorer balance is able to weigh samples down to 20mg (in real world cases when measuring in a controlled environment and observing good weighing practices, down to 10mg after a proper qualification), while maintaining compliance with USP41 minimum weight standards. In addition to their outstanding weighing performance, the OHAUS Explorer semi-micro balances contain features that help ensure maximum weighing accuracy, such as anti-static devices.  Static buildup can be a source of substantial error when weighing small samples.  The Explorer Series also contains several features that make the balances easy to use and suitable for use in regulated environments, such as automatic draftshield doors, a large color touch screen display,  an intuitive user interface, and features that aid in GMP/GLP compliance and provide measurement traceability.